1. Field of the Invention
The present invention relates generally to devices for counting the number concentration of fine particles suspended in air or gas, and, more particularly, pertains to that class of devices termed condensation nucleus counters. The primary purpose of the device is for monitoring air quality within designated areas, but it is useful in a variety of other scientific and industrial applications for counting the number of particles in the atmosphere.
2. Discussion of Related Technology
With the line widths of semiconductor devices going below 1 micrometer and the layer thickness below 0.1 micrometers, there is an increasing need for detecting and controlling submicrometer particles in the clean areas where the devices are manufactured. Particle contaminants much smaller than the line geometries can destroy the device and reduce production yield.
A common technique for detecting airborne particles is with an optical particle counter. Sample air flows into the device and intersects with a controlled beam of light. The particles in the sample air scatter the light in proportion to their size, shape and refractive index. The scattered light is collected onto a photoelectric device and converted into an electrical signal. The electrical signal is typically calibrated and processed to give the size distribution and number concentration of the particles. The theoretical lower limit of detection is approximately 0.05 micrometers diameter. Knollenberg R. G. and R. Luehr: Open Cavity Laser "Active" Scattering Particle Spectrometry, Fine Particles, edited by B. Y. H. Liu, Academic Press, Inc., New York (1976) pp. 669-696. The practical lower limit is closer to 0.1 micrometers diameter.
Another technique, useful for identifying and counting extremely small particles, is condensation nucleus counting. In this method, a liquid condenses on the particle, thus enlarging the target and thereby simplifying its identification. The theoretical lower limit of detection for a condensation nucleus counter is about 0.003 micrometers. Stolzenburg, M. R. and P. H. McMurry: Counting Efficiency of an Ultrafine Aerosol Condensation Nucleus Counter: Theory and Experiment, Aerosols:Formation and Reactivity, 2nd Int. Conf. Berlin, Pergamon Journals Ltd., Oxford, Great Britain (1986) pp. 786-789.
The literature describes three basic techniques for condensing vapor onto small particles for use in a counting instrument: (1) adiabatic expansion, (2) diffusional thermal cooling, and (3) two-flow mixing.
The first condensation technique (Aitken, J.: On the Number of Dust Particles in the Atmosphere. Proc. Royal Soc. Edinburgh, 35 (1888) uses adiabatic expansion of a water-saturated air-sample to cool and condense water onto small particles. In the Aitken method, the droplets are counted as they fall onto a grid. Later improvements to the technique include using light and electrical photodetectors to measure the light attenuation from the cloud formation, use of both under and overpressure systems, and automation of the flow system and adiabatic expansion. All of the presently available commercial instruments use water as the condensing fluid and operate in a pulsed flow fashion. The lowest particle concentration sensitivity obtainable with this method is approximately 100 particles/cm.sup.3.
The second condensation technique (Madelaine and Reiss, United Kingdom Pat. No. 1,422,188 and Sinclair, U.S. Pat. No. 3,806,248) utilizes continuous-flow thermal-diffusional cooling of an alcohol vapor. The air sample is saturated with alcohol vapor as it passes over a heated pool of liquid alcohol. The vapor-aerosol mixture is then cooled by thermal diffusion from the cold walls of the condenser tube. The vapor supersaturates and condenses on the surface of the particles so as to form larger droplets. The droplets are detected in a conventional optical particle detector or by photo attenuation of a light beam. A commercial instrument (Agarwal, J. K. and G. J. Sem: Continuous Flow, Single-Particle-Counting Condensation Nucleus Counter. J. Aerosol Sci., Vol. 11, No. 4, (1980) pp. 343-357) employs two modes of concentration measurement to cover the range of particle concentrations from 0.01 to 10.sup.7 particles/cm.sup.3. For particle concentrations of less than 1000 particles/cm.sup.3, the optical detector counts individual particle-produced pulses as they pass individually and sequentially through the beam of light. For higher particle concentrations, the total amount of light scattered, measured as the DC electrical signal from the photodetector, is calibrated to a known concentration using the electrical calibration technique (Liu, B. Y. H. and D. Y. H. Pui: A Submicron Aerosol Standard and the Primary Absolute Calibration of the Condensation Nucleus Counter. J. Colloid Int. Sci., Vol. 47 (1974) pp. 155-171).
The third condensation technique (Kohsaka, Nonaka, and Tachibana, U.S. Pat. No. 4,449,816) turbulently mixes two aerosol-laden vapors, one hot and one cool, which causes rapid vapor supersaturation and condensation on the particles. The droplets are counted with a conventional optical particle detector. The two flows are continuous. The concentration range is similar to the previously discussed diffusional-cooling technique.
The present invention, a condensation nucleus counter, is a device that detects and counts the number concentration of small airborne particles predominantly in the submicrometer size range.
The device is not limited to air, but is suitable for many other gases as well. Condensation formed on the particles from a supersaturated vapor enlarges the particle size and forms liquid droplets. The droplets are detected with a light scattering technique similar to that used in an optical particle counter. The lower detection limit of the present invention is a particle of approximately 0.014 micrometer diameter. The invention uses a similar principle of operation to the continuous-flow thermal-diffusional technique. The unique features of this invention have improved the performance, reduced the size, increased reliability, stability, and ruggedness, and provide for contamination-free operation in clean environments.